1. Field of the Invention
This invention relates to the field of electrical solenoids and more particularly pertains to solenoids that control the operation of a hydraulic valve through the application of a force whose magnitude varies with the magnitude of electrical current through the solenoid coil.
2. Description of the Prior Art
In electrical solenoids of the type used currently, when the coil is energized, induced magnetic flux passes in a magnetic circuit that includes a non-working air gap and conductive metal. The configuration of the working air gap determines in part the magnitude of the force produced by the magnetic field in relation to the current in the coil and applied to an actuator. When such a solenoid is used to produce a force that is proportional to current only and independent of the position of the actuator, the force produced is a function of the magnetomotive force (MMF) of the working air gap and the rate of change of the air gap permeance per unit change in the effective air gap. The permeance change includes not only the working air gap but also the radial leakage effects between the plunger and the outer shell.
It is difficult to evaluate analytically the magnitude of the force because the working gap MMF is dependent on the permeability of the metal part, which changes with flux density. Furthermore, the change in air gap permeance changes in a complicated way that can be expressed only graphically.
A variable force solenoid for regulating pressure so that the regulated pressure is proportional to the current in the coil of the solenoid requires that the coil resistance be limited by the minimum battery voltage available. For example, if the solenoid is to be mounted in a transmission wherein the operating temperatures can approach 300.degree. F., and if the full scale current is to be 1 amp., the resistance of a coil is limited to approximately 500 ohms at room temperature, or 9 ohms at elevated temperature. Therefore, because the number of ampere turns of the coil is restricted, to obtain sufficient force from the solenoid, the design must be efficient.
A variable force solenoid must produce a force on the plunger or actuator element that varies only with the coil current and not with the displacement of the actuator. This requires that the correct relationship be established between the MMF of the working air gap and the change in the air gap.